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Direct Fire Heating

5 min read

A fire under an autoclave turns it into a small fired boiler, and it is only as safe as the water covering the heated metal, because a wall that boils dry or is coated with scale overheats and fails at a pressure far below its rating. Charge a measured amount of water, shield the seams and gasket from the flame, plan how to pull the fire at once, and size the relief valve for the biggest fire the grate can hold.

Part of Heat Source

Heating an autoclave with wood, charcoal, coal or gas burning under it: what protects the wall, how the water is managed, and where it goes wrong.

Water Is The Only Thing Keeping The Wall Cool

The carrier’s simplest method places the autoclave on a fire grate with wood, charcoal or a gas burner under it. The steel wall where the flame touches is cooled only by the water on the other side. While that wall is wet, it stays close to the water’s temperature. Once the water level falls below the heated surface, or a layer of scale insulates the metal, the wall runs far hotter than the steam, loses strength, and can bulge or split under a pressure it would normally carry easily.

The Boiler Construction article gives the same warning in its own setting: a 3 mm scale layer insulates the metal from the water and creates hot spots, so soft water and regular draining matter, and firebrick should line the firebox wherever flame meets metal to prevent localised overheating.

The Carrier’s Water Charge Can Be Gone In One Purge

The carrier’s procedure is to put 2-5 cm of water in the bottom, load the rack, seal the door, heat, and vent for five minutes at atmospheric pressure before closing the vent to build pressure. Check what the five-minute vent can cost in water. All of the numbers below are assumptions to show the scale of the problem, and not a measurement of any vessel:

  • Take the smallest carrier chamber, 300 mm in diameter. A 2 cm layer of water is π × 0.15² × 0.02 = 0.00141 m³, or about 1.4 litres; a 5 cm layer is about 3.5 litres.
  • Take an illustrative 10 kW of fire delivered to the vessel for the 300 seconds of the vent: 10 kW × 300 s = 3,000 kJ.
  • Boiling water at atmospheric pressure takes about 2,257 kJ/kg (recalled steam-table value, flagged for verification). If all of it went into steam, 3,000 ÷ 2,257 = about 1.3 kg of water, or 1.3 litres, would leave through the vent.

That is an upper bound, because much of the heat goes into warming the steel and the load. But it shows that a strong fire during the purge can carry away nearly all of a 2 cm water layer. So the water charge must be measured in, not guessed, and chosen with a strong fire in mind.

What You Need

  • A grate or stand with a firebrick or plate shield between flame and the vessel’s seam welds, nozzles and gasket.
  • A measured water charge, poured from a marked jug and recorded.
  • A way to check the water level where practical: a water-level gauge or try-cocks as in the Water Level Gauge article. If the vessel is fully sealed and has none, set a cycle time limit beyond which the fire must be pulled.
  • Soft water. Rain is ideal, since hard water scales the heated surface.
  • A means to remove the fuel quickly: a raked-out firebox, a damper or a pull-out tray.
  • A relief valve sized for the strongest fire (Pressure Relief Valve).

Method: Running A Fired Autoclave

Step 1: Measure the water. Pour in the recorded charge and write it in the log.

Step 2: Check the seams and gasket are out of the flame path. The shield should take the heat, not the weld.

Step 3: Light the fire small, and build it gradually to the heat you need. Do not start with a roaring fire.

Step 4: Purge the air with the vent open until a steady stream of steam comes out, then close it, watching the gauge and thermometer together.

Step 5: Hold the cycle by managing the fire, adding fuel in small amounts. A bigger fire never helps once the temperature is reached; it only adds steam for the relief valve.

Step 6: At the end of the hold, pull the fuel. Do not wait for it to die.

Step 7: After cooling, drain the water and dry the chamber, particularly the bottom. A damp bottom corrodes the heated, stressed part first.

Common Mistakes

  • Starting with a hot fire. It heats the wall unevenly and boils off the water layer faster than the air can be purged.
  • Letting flames lick the weld seam. Hot seams lose strength and crack.
  • Using hard water and never draining. Scale builds up on the very surface that most needs cooling.
  • Leaving the fire to burn out after the hold. It keeps making steam.
  • Gas burner leaks searched for with a flame. Use soapy water on joints and keep the area ventilated; never test for gas with a lit match or candle.

Safety

A boiled-dry fired vessel can fail at far below its rated pressure

Never leave a fired autoclave unattended. Never run it with an untested relief valve. If you think the water has gone low, pull the fire and let the vessel cool completely before opening it; do not add cold water to a hot dry vessel, which can crack the metal and flash to steam.

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